Many consumer and commercial appliances, including furnaces, water heaters, ovens, etc., include gaseous fuel burners. These appliances typically operate by providing a controlled gaseous fuel flow valve and an ignition source for igniting the flow of gaseous fuel in the burner housing. To ensure safety of operation, these appliances typically also include a flame sensor that is used to detect the presence or absence of flame in the burner housing. The output of this flame sensor may be used by the appliance controller or other circuitry to control the flow of gaseous fuel through the gaseous flow valve, to control the ignition source (such as where electronic spark, hot surface, etc. ignition are used), and to control a purge fan if one is provided. Such controls are necessary to prevent a condition where gaseous fuel is continued to be delivered to the burner housing without being combusted. If such a case were allowed to continue, the accumulation of unburned gaseous fuel in the burner assembly could result in a potentially explosive condition. Further, such control also allows for the diagnosis of potential problems and the identification of the need for cleaning or maintenance on the burner based upon the quality of the flame sensed therein.
While various methods of flame sensing are known in the art, including optical and pyrometer type sensors, a preferred method of sensing flame in consumer and commercial appliances such as those identified above and others is known as the flame rectification method for sensing flame. Indeed, many gas control safety standards written for such applications by, e.g. the American Gas Association now the Canadian Standards Association, specify that the flame rectification methodology of flame sensing be employed. The phenomenon of flame rectification is well known in the art. Specifically, it is known that the outer cone of a flame is ionized and can conduct electricity. Under the principle of flame sensing by flame rectification, two electrodes of different size are placed in contact with this outer envelope of the flame. These two differently sized electrodes are then connected to a circuit that supplies an AC voltage thereacross. In this configuration, the current that flows through the flame tends to flow only in one direction, from the smaller electrode to the larger electrode.
Recognizing that the presence of a flame will allow essentially DC current to flow therethrough, various circuits have been developed that allow for the sensing of both the presence and quality of the flame. These circuits may be broadly classified in one of two technology areas. The first area, to wit analog circuits, employ junction field effect transistors (JFETs). In such analog circuits, a JFET is configured as an amplifier and produces a negative voltage that is somewhat proportional to the flame current. Essentially, the JFET transistors are used to provide a high impedance buffer from the flame sense circuit to the appliance control electronics.
Unfortunately, such prior analog circuits do not provide an accurate measure of the flame current, and are particularly sensitive to normal variations of the component parameters. Two such parameters of a JFET that have a significant impact on the effectiveness of such circuits are the input to output gain and the gate turnoff threshold. Further, these parameters have wide variations with normal production and temperature tolerances. In such conventional circuits, these variations produce inaccuracies in the flame sense circuit. Even when JFETs are specifically manufactured and selected in production for a narrower range of these parameters, the remaining variations still significantly affects the circuit performance. As a result, these analog circuits suffer from poor accuracy.
The second class of flame sense circuits utilizing the flame rectification methodology includes digital circuits. Unfortunately, the typical digital flame sense circuit also uses a JFET transistor. In these digital circuits, the time required for the flame current to charge a capacitor at the input terminal of the JFET is measured. The voltage pulse width at the output terminal of the JFET is somewhat proportional to the flame current. While such digital circuits have been designed to reduce the poor performance effects of the JFET transistors in the analog circuits, the digital circuits still suffer from poor accuracy. Additionally, their added complexity also increases the system cost, reduces reliability, and does not allow for a straightforward measurement of the flame current with common laboratory instruments. Further, while the digital circuits utilize various algorithms in an attempt to compensate for the JFET transistor inaccuracies, the algorithms cannot accurately adapt for all of the various transistor inaccuracies, appliance parameters, specific electrode sizes, type of gas, etc. in a cost-effective reliable circuit that may reliably be employed for such appliances.
There exists, therefore, a need in the art for a simple, reliable, and accurate flame sense circuit that not only provides reliable detection of the presence of a flame, but also provides a simple method of determining the strength and/or quality of the flame.